DARLINGTON - Almost 500 million years ago, a warm, shallow saltwater sea covered what is now Wisconsin. Large, shelled creatures dominated those seas, using jet propulsion to move around.

These creatures, called nautiloids - a type of cephalopod - are related to the modern squid and octopus. But unlike their modern cousins, they had a shell resembling a giant unicorn horn, some growing to almost 20 feet long.

"A good way to think about it is like if you took an ice cream cone - a big one - and stuck an octopus in the end of it," said Jim Kostohrys, who lives between Darlington and Mineral Point. "And you'd have yourself a cephalopod."

A nautiloid would fill the hollow chambers within its horn-like shell with water or gas in order to raise or lower itself in the water, like a submarine, Kostohrys said. It would also suck water into a pouch and then blow it back out to propel itself backward, point first.

After about 150 million years, the straight-shelled cephalopod died out - but not without leaving something behind.

Kostohrys, a fossil collector and natural history enthusiast, came across the tip of a nautiloid fossil coming out of a rock in the Mineral Point area. He started digging it out, knowing it would either taper off to a point or grow into the opening of the shell.

"I was lucky. It got bigger and bigger and bigger, and I kept getting more and more and more out of it," he said.

Kostohrys spent about a year and a half excavating and piecing together sections of the roughly 15-foot-long fossil. A large chunk was in a roadcut and another part was from a local quarry. Most of the pieces came from the same location, "so I know it came from the same animal," he said. And they all fit together.

Not all of the pieces came from the same cephalopod, he said, but the creatures are mostly the same. Kostohrys said he can determine the length of the animal by extending the slope of one piece of the shell until it comes to a point. Then he can fill in the missing pieces from similar cephalopod fossils.

"I don't want to say Frankenstein," Kostohrys said. "There are a couple little sections there that didn't really belong to the same animal, you know, but I filled them in."

He can also tell where the large end of the fossil is, because as the "living chamber" - where the creature's main body lived - it leaves different markings on the rock. Kostohrys pointed out the marks and shells in the living chamber left from scavengers that feasted on the nautiloid after it died.

Another two feet of the living chamber are still embedded in the rock where Kostohrys found the fossil, but he said he hasn't been able to get it out yet. The pieces he currently has measure around 12 feet long when laid end to end.

The rest of the fossil looks different from the living chamber, with stripes marking the chambers inside the shell.

Kostohrys, 65, has been collecting fossils for at least 50 years and has been making scientific drawings for the Field Museum of Natural History in Chicago for about 20 years.

"One of the things that this has taught me, you know, with fossils and stuff, is keen powers of observation," he said. "You gotta really look. Because you'll miss it if you don't look at it, then look at it again and then look at it under a magnifying glass."

He said this after pointing out the fine, barely-noticeable ribs on a fossilized leaf from his collection. Observation has helped him with his scientific drawings, which he said are precise "to the dot."

His passion for fossils started when he was about 10 years old. Kostohrys would sneak into nearby quarries - "hop a fence, climb under a big, high barbed wire" - and look for fossils, which he took home with him. "I was a geeky little kid," he said.

(For the sake of safety and legality: If anyone decides to go fossil hunting now, Kostohrys recommends they take a friend along, make sure others know where they'll be, and always ask for permission from the landowner.)

Kostohrys credits one adult in particular for getting him interested in fossils and natural history. It was a friend's father, a "forest ranger kind of guy," who took Kostohrys under his wing.

"He opened up a whole world for me," Kostohrys said.

And he enjoys doing the same for kids today.

"I like to have kids learn, because someone took the time with me when I was a little guy," he said. "Gave me something that interested me all my life."

He gives talks on fossils and will be displaying a piece of his large nautiloid fossil at the Bluebird Nest Nature Center in Darlington Wednesday for its Cephalopod Day. The event will be from 1 to 5 p.m.

Over the years, Kostohrys amassed a large collection of "thousands and thousands" of fossils, most of which he donated to the Weis Earth Science Museum at University of Wisconsin-Fox Valley. It took three trucks and lots of student help to haul the collection.

"And there was no junk in it," he said. "It was really first-class stuff."

His nautiloid may receive the same treatment.

"I'd be more than happy to just donate it," Kostohrys said, suggesting somewhere like Wisconsin's Cave of the Mounds, so kids could see what the full, excavated fossil looks like. A 9-foot-long cephalopod fossil is already partially exposed in the cave's rock wall.

Straight cephalopod fossils can be found all over the world, wherever there's rock from the Ordovician period, Kostohrys said. The Ordovician period was between 400 and 500 million years ago, during the Paleozoic Era.

Rock around here, like the one that contained Kostohrys' nautiloid fossil, has been dated at about 500 million years old through radioisotope dating, Kostohrys said.

Other fossils from the Ordovician period can be seen on some walls inside the Capitol building in Madison. They feature coiled shells.

Only one creature alive today represents the same type of animal as the Ordovician nautiloids, but it has a coiled shell, according to Kostohrys. It's called the chambered nautilus and lives near the Philippines.

"It's kind of a living fossil," he said.